Speaking broadly, the diameter of the crater is a measure of the violence of the explosions within the chimney. A single series of short and violent explosive eruptions builds a low and broad cinder cone. A long-continued succession of moderately violent explosions, on the other hand, builds a high cone with crater diameter small if compared with the mountain’s altitude, and the profile afforded is a remarkably beautiful sweeping curve (Fig. 121). Toward the summit of such a cone the loose materials of which it is composed are at as steep an angle as they can lie, the so-called angle of repose of the material; whereas lower down the flatter slopes have been determined by the distribution of the cinder during its fall from the air. When one makes the ascent of such a mountain, he encounters continually steeper grades, with the most difficult slope just below the crest. [Illustration: FIG. 122.—A series of breached cinder cones where the place of eruption has migrated along the underlying fissure. The Puys Noir, Solas, and La Vache in the Mont Dore Province of central France (after Scrope).] =The composite cone.=—The life histories of volcanoes are generally so varied that lava domes and the pure types of cinder cones are less common than volcanoes in which paroxysmal eruptions have alternated with explosions, and where, therefore, the structure of the mountain represents a composite of lava and cinder. Such composite cones possess a skeleton of solid rock upon which have been built up alternate sloping layers of cinder and lava. In most respects such cones stand in an intermediate position between lava domes and cinder cones. [Illustration: FIG. 123.—The _bocca_ or mouth upon the inner cone of Mount Vesuvius from which flowed the lava stream of 1872. This lava stream appears in the foreground with its characteristic “ropy” surface.] Regarded as a retaining wall for the lava which mounts in the chimney, the cinder cone is obviously the weakest of all. Should lava rise in a cinder cone without an explosion occurring, the cone is at once broken through upon one side by the outwelling of the lava near the base. Thus arises the characteristic _breached_ cone of horseshoe form (Fig. 122). [Illustration: FIG. 124.—A row of parasitic cones raised above a fissure which was opened upon the flanks of Mount Etna during the eruption of 1892 (after De Lorenzo).]